CHECKS: Remove folder .future/ created by R CMD check

[HenrikBengtsson]

R-devel/CRAN now reports (https://cran.r-project.org/web/checks/check_results_future.batchtools.html):

using R Under development (unstable) (2019-06-05 r76673)
using platform: x86_64-pc-linux-gnu (64-bit)
using session charset: UTF-8
...
checking PDF version of manual ... OK
checking for non-standard things in the check directory ... NOTE
Found the following files/directories:
  .future
DONE